a new generation of sensors based on extraordinary optical

A New Generation of Sensors Based onExtraordinary Optical TransmissionREUVEN GORDON,† DAVID SINTON,‡ KAREN L. KAVANAGH,§

AND ALEXANDRE G. BROLO*,⊥†Department of Electrical and Computer Engineering, University of Victoria,

P.O. Box 3055, Victoria, B.C. V8W 3P6, Canada, ‡Department ofMechanical Engineering, University of Victoria, P.O. Box 3055, Victoria,

B.C. V8W 3P6, Canada, §Department of Physics, Simon Fraser University,8888 University Drive, Burnaby, B.C. V5A 1S6, Canada, ⊥ Department of

Chemistry, University of Victoria, P.O. Box 3065, Victoria,B.C. V8W 3V6, Canada

RECEIVED ON MARCH 4, 2008

C O N S P E C T U S

Plasmonic-based chemical sensing technol-ogies play a key role in chemical, biochem-

ical, and biomedical research, but basicresearch in this area is still attracting interest.Researchers would like to develop new typesof plasmonic nanostructures that can improvethe analytical figures of merit, such as detec-tion limits, sensitivity, selectivity, and dynamicrange, relative to the commercial systems.They are also tackling issues such as cost,reproducibility, and multiplexing with the goalof providing the best plasmonic-based plat-form for chemical analysis.

In this Account, we will describe recentadvances in the optical and spectroscopic properties of nanohole arrays in thin gold films and their applications for chem-ical sensing. These nanostructures support the unusual phenomenon of “extraordinary optical transmission” (EOT), that is,they are more transparent at certain wavelengths than expected by the classical aperture theory.

The EOT is a consequence of surface plasmon (SP) excitations; hence, the resonance should respond to the adsorptionof organic molecules. We explored this effect and implemented the integration of the arrays of nanoholes as sensing ele-ments in a microfluidic architecture. We then demonstrated how these devices could be applied in biochemical affinity tests.Arrays of nanoholes offer a small sensing footprint and operate at normal transmission mode, which make them more suit-able for miniaturization. This new approach for SPR sensing is more compatible with the lab-on-chip concept and offersthe possibility of high-throughput analysis from a single sensing chip.

We explored the field localization properties of EOT for surface-enhanced spectroscopy. We could control the enhance-ment factors for SERS and SEFS by adjusting the geometry of the arrays. The shape of the individual nanoholes offers anotherhandle to tune the enhancement factor for surface-enhanced spectroscopy and SPR sensitivity. Apexes in shaped nanostruc-tures function as optical antennas, focusing the light at extremely small regions at the tips. We observed additional sur-face enhancement by tuning the apexes’ properties. The extra enhancement in these cases originated only from the smallnumber of molecules in the apex regions.

The arrays of nanoholes are an exciting new substrate for chemical sensing and enhanced spectroscopy. This class ofnanomaterials has the potential to provide a viable alternative to the commercial SPR-based sensors. Further research couldexploit this platform to develop nanostructures that support high field localization for single-molecule spectroscopy.

Vol. 41, No. 8 August 2008 1049-1057 ACCOUNTS OF CHEMICAL RESEARCH 1049Published on the Web 07/08/2008 www.pubs.acs.org/acr10.1021/ar800074d CCC: $40.75 © 2008 American Chemical Society

1. Introduction

The recent advances in nanofabrication methods1,2 and nano-

particle synthesis3,4 are leading to the advent of a variety of

new nanostructures with unusual physical and chemical prop-

erties. Nanostructured free-electron metals are particularly

interesting, because they can be resonantly excited by visi-

ble light to produce surface plasmon (SP) oscillations.5 SPs are

collective electronic oscillations that lead to surface-bound

electromagnetic fields. They are categorized into two class-

es: propagating surface plasmons or surface plasmons polari-

tons (SPPs)3,5 and localized surface plasmons (LSPs).6 In most

cases, both SPP and LSP resonances are present and contrib-

ute to the optical characteristics of the nanostructure.7 The

manipulation of the properties of SPs by tailoring the geomet-

ric parameters of the nanostructures is the heart of the new

field of plasmonics.5 The plasmonic approach offers an excit-

ing alternative for the manipulation of light in the subwave-

length regime.8 The potential for the development of

plasmonic-based technologies is a major driving force in this

area, and several promising applications have been suggested.

For instance, plasmonic wave-guides could allow the process-

ing of a large amount of information at light speed,3 super-

lensing effects offers a new route to beat diffraction limitations

in imaging systems,9 and the inherent subwavelength con-

finement of the plasmonic field provides a viable alternative

for nanolithography.10 All these examples of application have

the potential to bring revolutionary advancements in informa-

tion processing, device fabrication and imaging technologies.

On the other hand, the plasmonics revolution has already

arrived to the field of chemical sensing, where SP waves are

widely used in important fundamental and practical applica-

tions.2 The capability of SPs to monitor surface-binding events

plays a key role in chemical, biochemical, and biomedical

research.11 The market has responded to the demand for reli-

able plasmonic devices for chemical sensing by providing sev-

eral options for commercial systems that operate based on the

principle of SP excitation.11

There are basically three approaches for SP-based chemi-

cal sensing. The first relies on the dependence of the surface

plasmon resonance (SPR) on the dielectric constant at the

metal-dielectric interface. The SPR is then sensitive to the

changes in refractive indexes provoked by molecular adsorp-

tion, a widely explored property by the bioanalytical commu-

nity to monitor binding events.11 The second approach

explores the red shift in the LSP absorption of metallic nano-

particles (NPs) due to aggregation to devise colorimetric

assays.12 The classical example for the technological applica-

tion of this approach is the Au nanoparticle-based pregnancy

test.13 Finally, the third approach explores the subwavelength

electromagnetic (EM) field localization that accompanies the SP

excitations for enhanced spectroscopy, such as surface-en-

hanced Raman scattering (SERS)14 and surface-enhanced flu-

orescence spectroscopy (SEFS).15 Enhanced spectroscopy

methods allow direct molecular identification and single-mol-

ecule detection limits.14

Plasmonic-based chemical sensing technologies are being

successfully applied and commercialized, and the basic

research in this area is still attracting much interest. The gen-

eral objective is to develop new types of plasmonic nanostruc-

tures that can improve the analytical figures of merit, such as,

detection limits, sensitivity, selectivity, and dynamic range,

provided by the commercial systems. Other issues, such as

cost, reproducibility, and multiplexing, are also tackled, with

the goal of providing the best plasmonic-based platform for

chemical analysis. Following these lines, our group has been

investigating the interesting phenomenon of light transmis-

sion through subwavelength holes (nanoholes) in gold films

and found that this class of substrate may provide a few

advantages over more common chemical sensing platforms.

In this Account, we will discuss our main contributions to this

particular field and propose an outlook for the future direc-

tions of this approach.

2. The Extraordinary Optical Transmission

It is intuitive that the amount of light transmitted through an

aperture in an opaque metal sheet should decrease with the

hole area. The physics behind this phenomenon has been

worked out more than 60 years ago by Bethe,16 who showed

that when the wavelength (λ) of the transmitted light is larger

than the hole diameter (d), the transmittance (T) is given by T

∝ (d/λ)4. A single hole in an infinitely thin slab was consid-

ered in Bethe’s formalism.16 An extension of his approach to

real metals17 and to arrays of holes has been reported.18 The

behavior of nanoholes in real metals with finite thickness was

subsequently explored.19

The idea that the amount of transmitted light decreases

with hole diameter according to Bethe’s law remained unchal-

lenged until 1998, when Ebbesen and co-workers realized a

seminal experiment on the light transmission through arrays

of nanoholes in Ag and Au thin films.20,21 Their result indi-

cated that the amount of transmitted light at certain wave-

lengths was much larger than predicted by the classical

aperture theory. They also demonstrated that there was more

light transmitted than the actual amount that impinged on the

New Generation of Sensors Gordon et al.

1050 ACCOUNTS OF CHEMICAL RESEARCH 1049-1057 August 2008 Vol. 41, No. 8

hole area; that is, the material seemed much more transpar-

ent than it should be. This unexpected phenomenon was

called extraordinary optical transmission (EOT). The effect was

observed for gold and silver films, but its magnitude

decreased when different metals were used. Moreover, the

peaks of maximum transmission were related to the dis-

tance between the nanoholes (periodicity). Considering all

these properties, it was suggested that EOT was due to the

excitation of surface plasmon resonances by a grating cou-

pling.20 Although this interpretation has since been chal-

lenged,22 the evidence from several laboratories seem to

consolidate the excitation of SP as the main contribution to the

EOT effect.23,24

An example of an array of nanoholes on Au that supports

EOT in the visible range is presented in Figure 1A. The array

in Figure 1A was fabricated using focused ion beam (FIB) mill-

ing. The EOT effect is illustrated in Figure 1B where the white

light transmission spectra through arrays of nanoholes of dif-

ferent periodicities are shown. The spectra in Figure 1B were

obtained using a microscope coupled to a miniature spectrom-

eter, as depicted in Figure 1C.25 Peaks in the white light trans-

mission (Figure 1B) correspond to the wavelengths that match

the EOT conditions for each structure. The amount of trans-

mitted light at the peaks far exceeded that expected from the

simple aperture theory.20

For periodic metallic nanostructures, the phase-matching

condition for SPP excitation coincides with the Bragg reso-

nances of the grating. At normal incidence, the wavelength of

SPP resonance (λSP) from an array of nanoholes with square

lattice symmetry can be estimated using20

λSP(i, j) )p� εdεm

εd + εm

√i2 + j2(1)

where p is the lattice constant (periodicity) of the array, i and

j are integers that define the scattering orders of the array, εd

and εm are the real part of the dielectric constants of the adja-

cent medium and the metal, respectively. Equation 1 is for SPs

on a smooth metal-dielectric interface, and it does not

directly apply for meshes. The EOT phenomenon from metal-

lic meshes and its applications have been recently reviewed

elsewhere.27

The zero-order (normal incidence) transmission spectra of

arrays with different periodicities (Figure 1B) shows EOT

slightly red-shifted from that expected from eq 1. Another

interesting feature from the EOT spectra shown in Figure 1B

is the asymmetry around the transmitted peaks. The mini-

mum in the transmission curves, marked with asterisks in Fig-

ure 1B, correspond to a diffraction phenomenon known as

Wood’s anomaly. A Fano analysis has been suggested to

explain the observed red shift of the SP resonance relative to

eq 1 and the line-shape asymmetry in the transmissions.28

The Fano treatment takes into consideration interfering reso-

nant and a nonresonant contributions to the phenomenon. In

the specific case of nanohole arrays, the interference between

the direct transmission through the nanoholes (incoherent

path) and the SP-mediated transmission (coherent path)

accounts for the resonance shift and asymmetry observed

experimentally in EOT.28 On the other hand, recent experi-

ments using two stacked micromeshes, which eliminate the

direct transmission path, still presented asymmetric transmis-

sion peaks.29

EOT was also observed for single holes, and contributions

from LSPs play a key role in these cases.19,30 The light trans-

mission depends strongly on the hole shape. For instance, lon-

gitudinal and transverse LSP modes can be individually

excited in rectangular holes by controlling the electric polar-

ization of the incident light. It has also been found that the cut-

off wavelength for the transmission increases as the width of

the rectangular holes is reduced, which is another counterin-

tuitive result from nanoholes in metals.31 The effect of the

hole shape on the transmission is also observed from arrays

of nanoholes.32,33 Another interesting property related to EOT

from single holes is the “beaming” of the transmitted light,19

which is a preferential direction for the emergent light beam

FIGURE 1. (A) Scanning electron micrograph (SEM) of an array ofnanoholes in a gold film; B) EOT spectra for three arrays withdifferent periodicitiessthe asterisk shows the position of theWood’s anomaly for one of the arrays; (C) experimental setup usedby our group to measure the EOT effect.26 The metal film isdeposited on a glass slide, and the gold side of the array isexposed to solvents and aqueous solutions delivered bymicrofluidics.


Vol. 41, No. 8 August 2008 1049-1057 ACCOUNTS OF CHEMICAL RESEARCH 1051

in contrast to the anisotropic distribution expected from sim-

ple diffraction. The beaming phenomenon is more pro-

nounced when a single nanohole is surrounded by a grating

structure.34 In the case of arrays of nanoholes, each hole acts

like a new point source. Therefore, collimated incident light is

recovered as a still collimated output.

3. Exploring EOT for Surface PlasmonResonance SensingThe EOT through arrays of nanoholes depends on the opti-

cal properties of the metal-dielectric interfaces through eq 1.

Therefore, the adsorption of molecules on the gold surface

should shift the transmission peak wavelength, leading to a

SP-mediated chemical sensor. Figure 2 shows a demonstra-

tion of the monolayer sensitivity of the SPR from arrays of

nanoholes reported by our group.25 The white light transmis-

sion spectrum through a clean array of nanoholes (bare Au

surface) presents a distinct resonant peak. The gold surface

was then modified by immersing the array of nanoholes in an

ethanoic solution of mercaptoundecanoic acid (MUA), lead-

ing to a characteristic red shift in the wavelength of maximum

transmission due to the changes in the dielectric properties of

the surface, as predicted by eq 1. Further modification of the

surface by a protein (bovine serum albumin, BSA) provoked an

additional wavelength shift. The spectrum characteristic of a

bare gold surface was recovered after the surface species were

removed by a plasma cleaning treatment.25

The results presented in Figure 2 are reminiscent of the

adsorption-induced shifts in the angle of minimum reflectiv-

ity in typical Kretschmann-configuration surface-plasmon res-

onance (SPR) experiments.11 SPR is one of the most widely

used tools for the study of binding in biochemical systems,

and Figure 2 shows that similar results can be obtained using

arrays of nanoholes.25 In contrast to the commercial SPR sen-

sors, the phase-matching condition for excitation of SPs in

zero-order transmission is given by the periodicity of the holes

(eq 1), and prism coupling is not required. This simplified opti-

cal setup is more suitable for miniaturization.

The combination of all the interesting properties of SP-me-

diated transmission, including the highly localized sensing

area and simple optical setup, render these types of substrates

ideal for the development of integrated biosensors in labora-

tory-on-chip devices. This concept was demonstrated by our

group,26 and the devices investigated are shown in Figure 3A.

Using standard microfluidics fabrication methods, we devel-

oped two configurations of the microfluidic layers, as shown

in Figure 3A. The sensitivity of the integrated arrays of nano-

holes was tested using standard sucrose solutions. The stan-

dard solutions were continuously flowed through the channels

while the whole white light transmission spectra were mea-

sured in real time using the setup shown in Figure 1C. The cal-

ibration curve, presented in Figure 3B, yielded a linear trend

within the range of refractive indexes investigated. The out-

put sensitivity of each array was calculated from the slope of

the calibration curves, averaging ∼333 nm/RIU (RIU ) refrac-

tive index units).26 The sensitivity found in our experiments

agreed well with the values predicted for grating-based SPR

devices35 and are similar to the sensitivities obtained from

sensing schemes based on metallic nanoparticles.36

The devices presented in Figure 3A were also used in affin-

ity tests involving the biotin-streptavidin system.26 Figure 3C

summarized the results of the affinity tests performed using

the arrays of nanoholes as sensor elements in the microflu-

idic device.

The results from Figure 3 demonstrate the potential of

using arrays of nanoholes as sensing elements in laboratory-

on-chip applications. The normal transmission setup allied to

the small footprint of the arrays offers an attractive opportu-

nity for miniaturization. An inexpensive device can be envi-

sioned where the detection of the whole transmission

FIGURE 2. The effects of molecular adsorption on EOT.25

FIGURE 3. (A) Schematic and SEM images illustrating thearchitecture of the microfluidic chips with embedded nanoholearrays; (B) calibration curve using sucrose solutions with differentconcentrations; (C) application of nanoholes for an affinity test.26



spectrum can be replaced by the measurements of changes in

the amplitude of the transmission from a single laser source,

a concept that has been reported already.37,38

The sensor output sensitivity obtained using the devices

presented in Figure 3A is an order of magnitude smaller than

the values from commercial angle-resolved SPR systems. This

is not a fundamental limitation to the application of arrays of

nanoholes for SPR sensing. It is important to point out that the

sensing area of the arrays of nanoholes is much smaller than

is typical of SPR arrangements; hence, the magnitude of the

wavelength shift from our experiment originates from a

smaller number of molecules. These considerations have been

taken into account by Stark et al.,37 who had demonstrated

better sensitivity and resolution than commercial SPR devices

by measuring the amplitude variations of a laser line through

a short ordered nanohole array. SPR resolution on the order

of commercial devices was also reported by Tetz et al.39 In

that case, they used crossed polarizers to select between the

coherent and the incoherent contributions to the EOT (these

are the contributions that lead to the Fano line shape observed

in Figure 1B). This scheme allows the recovery of a symmet-

rical line shape for the transmission, which provides better

sensitivity.39

4. Nanohole-Enhanced Spectroscopy

The results from Figure 3 show the potential for the applica-

tion of arrays of nanoholes in biomedical sensing and labo-

ratory-on-chip devices. Since the SP fields are tightly confined

to the interface, another powerful approach for chemical sens-

ing is to take advantage of this localization to produce

enhanced molecular spectroscopy. The EM localization is

known as the main contribution to SERS and SEFS.14,15 The

application of SERS and SEFS as a highly sensitive analytical

tool has been described for both random and patterned

substrates.14,40 Nanoholes in noble metals are a promising

platform for enhanced spectroscopy because they not only

confine the EM field but also restrict the analyte to very small

volumes. Moreover, in contrast to colloidal particles dispersed

on glass, the hot spots responsible for the field enhancement

are specifically organized by the nanofabrication of the grat-

ing.41 SERS measurements from a probe molecule, oxazine

720, adsorbed on arrays of nanoholes in gold films were

obtained by our group.42 The experimental geometry (for-

ward scattering) is presented in Figure 4A. The Raman scat-

tering of oxazine 720 was enhanced through the EOT of the

laser excitation. The relationship between the SERS responses

and the periodicity of the arrays was investigated, and it is

shown in Figure 4B.42 The inset in Figure 4B shows the trans-

mission spectra of three arrays with different p-values. The

dashed line corresponds to the laser excitation, fixed at 632.8

nm, and the arrow is to emphasize the position of the SP res-

onance relative to the laser line. It is clear that a strong

enhancement is observed when the laser energy is closer to

the SP excitation (red spectra), and the enhancement

decreases as the separation between the laser energy and the

EOT resonance increases.

The enhancement of the Raman signal was estimated to be

of the order of 105 for oxazine 720 relative to the normal

Raman of liquid benzene. This enhancement factor, however,

includes resonance Raman (RR) contributions, since the laser

excitation at 633 nm is within an electronic absorption of

oxazine 720. Attempts to obtain SERS from other species, such

as rhodamine 6G and pyridine, adsorbed on the nanoholes in

gold were not successful, indicating that the additional RR con-

tribution was essential for the observation of the spectra in Fig-

ure 4B. A quantitative evaluation of the enhancement factor

from nanoholes in silver was realized by Reilly et al.43 An

enhancement factor of 102 due exclusively to the SP excita-

tion through nanoholes was obtained. SERS from randomly

distributed nanoholes in gold films and normal Raman from

single nanoholes in metal were also reported.44,45

Infrared (IR) spectroscopy is another vibrational tool that

provides complementary information to the Raman scatter-

ing. It has been shown that EOT through Ni meshes enhances

the IR absorption from adsorbates.46 A 2 orders of magnitude

enhancement in the IR intensities was observed due to the

increased interaction time provided by the trapped light at the

interface in the form of SP waves.

Although vibrational spectroscopy (Raman and IR) offers

unique spectral characteristics of a chemical species, several

of the current biomedical assays utilize fluorescence probes.

Therefore, there is a great interest in exploring the applicabil-

ity of the arrays of nanoholes as substrates for SEFS,47–51 aim-

FIGURE 4. (A) Schematic of the forward scattering setup; (B)nanohole-enhanced Raman scattering from oxazine 720 fromnanoholes with different periodicities (p). The inset shows thetransmission spectra of the arrays: (a) p ) 560 nm; (b) p ) 590 nm;(c) p ) 620 nm.42



ing at further improvement on the sensitivity and limit of

detections of fluorescence-based biomedical tests.

The geometry of our SEFS experiments was similar to the

one presented in Figure 4A, but now the nanoholes were cov-

ered with a polystyrene film doped with the dye.47 The poly-

styrene is necessary because a film thicker than a monolayer

is required for SEFS, due to the well-known fluorescence

quenching experienced by emitters in close proximity to

metallic surfaces. The fluorescence emissions from films of

similar thickness but from arrays of nanoholes with different

periodicities are shown in Figure 5A. The amount of fluores-

cence emissions from the arrays was always larger than

expected from classical aperture theory when the conditions

for SP resonance were achieved.

Another very relevant aspect for chemical analysis that was

noted in our SEFS experiments47 is illustrated in Figure 5B.

The integrated fluorescence from each array was plotted

against the concentration of dye in the film. The slope of these

plots corresponds to the sensitivity from each array. The

results from Figure 5B clearly show that the normalized sen-

sitivity increased by more than three times when the condi-

tions for EOT at the excitation laser frequency were

achieved.47 This is a very important observation because it

demonstrates that not only the detection limit but also the

optical response to concentration changes are optimized by

the plasmonic structure.

5. Enhanced Spectroscopy and Sensingfrom Shaped NanoholesAnother interesting feature that can be explored for chemi-

cal sensing and enhanced spectroscopy is the dependence of

the EOT on the hole shape.7,33,52,53 The anisotropic structures

present selective polarization-dependent transmission. For

instance, we had shown that elliptical holes act as nanopolar-

izers, with preferential transmission when the light field is

polarized perpendicular to the major axis of the ellipse.52 Sim-

ilar effects have been observed for rectangular holes.53 The

line width of the transmission peak was also shown to depend

on the aspect ratio of rectangular apertures;53 therefore, struc-

tures with a sharper transmission band can be devised by geo-

metric optimization to increase sensitivity.

One interesting hole shape that has been explored by our

group for SERS and surface-enhanced second harmonic gen-

eration (SESHG) is the “double-hole” structure.33,54,55 In this

case, each single circular hole from a typical array is replaced

by two holes separated by a controllable distance, as shown

in Figure 6. The distance variation adds another degree of

freedom for geometrical optimization of the plasmonic prop-

erties. An interesting situation arises when the two holes over-

lap. In this case, shown in Figure 6C, two sharp apexes are

created. The apexes can act as an optical antenna, focusing

the electromagnetic field to a very small region at the sharp

FIGURE 5. (A) Emission from arrays of nanoholes coated with apolystyrene film (PS) with (a) p ) 553 nm in the absence of the dyein the PS film (background), (b) p ) 407 nm and 3 µM ofdye embedded in the PS film, (c) p ) 653 nm and 3 µM of dyeembedded in the PS film, and (d) p ) 553 nm and 3 µM of dyeembedded in the PS filmsthe inset shows the calculatedenhancement factor for all arrays investigated; (B) sensitivityobtained from plots of emission versus dye concentration in the PSfilm for arrays with different periodicities.47

FIGURE 6. SEM picture of double holes (d is the center-to-centerhole distance): (a) d ) 250 nm, hole diameter ) 200 nm; (b) d )210 nm, hole diameter ) 200 nm; (c) d ) 190 nm, hole diameter )200 nm; (d) array of double holes with 750 nm periodicity; (e)arrays of double holes with 550 nm periodicity.54



points at the apexes. Figure 7A shows a FDTD calculation of

the field distribution around a double hole with overlapping

holes.

It can be seen that the EM field is expected to be strongly

concentrated in the apex regions. Since a strong localized field

is the main requirement for enhanced spectroscopy, tunable

surface enhancement is expected from these structures. We

have obtained SERS from the double-hole structures with dif-

ferent hole distances. The dependence of the SERS efficiency

versus the spacing between the holes in the basis of the lat-

tice is shown in Figure 7B. It can be seen that, as expected

from the electromagnetic theory, the SERS signal maximizes

when the two holes overlap. The results from Figure 4 com-

bined with the observations from Figure 7 show that the dou-

ble-hole structure offers two degrees of freedom, shape and

periodicity, for electric field optimization and enhanced spec-

troscopy. It is important to emphasize that the optimized cen-

ter-to-center hole offered an additional increase in the SERS

that arises only from the molecules within the apex region. In

other words, the number of species that are generating this

extra enhancement is very small. Therefore, these antenna

structures are promising for the detection of ultralow amounts

of adsorbed species by SERS.

Beyond their application in enhanced spectroscopy, arrays

of shaped nanoholes can also be optimized for SPR detec-

tion. Recently, it has been shown that the double-hole struc-

tures provide extra sensitivity to the response to refractive

index changes.56 Similar observations were obtained from the

application of slits in gold films for SPR sensing.57

6. Outlook and Conclusions

In this Account, we described our contributions to the appli-

cation of arrays of subwavelength holes in chemical sensing

and enhanced spectroscopy. The arrays of nanoholes present

significant advantages over other sensing schemes. They are

ideal SPR sensors for a microfluidics environment, and their

sensitivity may rival the performance of commercial SPR sys-

tems. The planar platform and transmission optics render this

class of substrates ideal for multiplexing in microarray for-

mat. Microarrays, consisting of nanohole arrays as sensor ele-

ments, for multiple detection and high-throughput analysis

seem to be the next step for this technology, and some pre-

liminary examples have been proposed.38

In terms of applications in enhanced spectroscopy, arrays

of circular nanoholes are suitable for SERS, SEFS, and SESHG.

The enhancement factor obtained, however, is not as high as

that observed from disordered structures. However, the

enhancement properties of the arrays can be improved by

using antenna structures. The double-hole structure, explored

by our group, is a good example. In this case, it was shown

that the geometry of the basis and the periodicity of the lat-

tice can be independently optimized for maximum enhance-

ment. The extra enhancement due to the antenna region

originates only from the molecules adsorbed in the apexes.

This indicates that this approach may be viable for the fabri-

cation of structures for single-molecule SERS.

Looking more into future applications, it is noteworthy that

nanoholes are unique among plasmonic nanostructures in

that they are fundamentally channels. In sensor applications

of nanohole arrays to date, these channels have been effec-

tively dead-ended, and thus the transport between the sen-

sor and the solution has been analogous to any other surface

patterned sensor. While microfluidics has aided in delivering

solutions to the arrays, the tremendous potential of the nano-

FIGURE 7. (A) FDTD calculation showing the enhanced field at theapexes of the double holes; (B) SERS as a function of the center-to-center hole distance within the double-hole basis. The array’speriodicity was 750 nm, and the diameter of the nanoholes was200 nm. The intensity of the 591 cm-1 band of oxazine 720 wasused.54



hole as a conduit has yet to be explored. We feel a flow-

through nanohole array sensing format, as shown in Figure 8,

presents several exciting possibilities: First, analyte transport

via diffusion on the scale of the nanohole is extremely rapid,

and thus flow-through greatly enhances the exposure of the

active area to the solution under test. Likewise, a flow-through

configuration could provide a model platform to study a range

of dynamic processes; analogous electrochemical measure-

ments in nanochannels that harness rapid diffusion on nanos-

cales show much promise.58 Second, flow-through nanohole

arrays, if connected in series with service microfluidics, would

achieve a solution sieving action that is unique among sur-

face-based sensing methods. Just as the nanophotonic struc-

tures enable intense confinement of the electromagnetic field,

flow-through enables sieving and nanoconfinement of an

adsorbed analyte in the same region. Third, while a single

nanohole would present a dominant flow resistance in an oth-

erwise microfluidic system, arrays serve to parallelize the resis-

tance and thus fluid handling and control can be compatible

between the parallel nanochannels and service microfluidics.

Fourthly, flow-through provides access to a previously unex-

ploited portion of the photonic structure and thus potentially

new insight into EOT and new applications. One such appli-

cation would be to immobilize a cell on the surface of a large

array of nanoholes and use access provided by the through-

nanoholes to deliver localized inputs and analyze localized

outputs on the subcellular scale.

One of the glaring limitations to the wide ranging applica-

bility of nanohole arrays in analytical chemistry is the serial

character of the FIB fabrication. However, several methods

for large area patterning have been described in the litera-

ture.24,59 These new lithographic methods offer a promising

avenue to overcome this limitation.

We gratefully acknowledge funding support for this work from

NSERC, CFI, and BCKDF. We also thank current support through

a NSERC SPG with the BC Cancer Agency and Micralyne Inc.

BIOGRAPHICAL INFORMATION

Reuven Gordon is an Associate Professor in Electrical and Com-puter Engineering at the University of Victoria. His recent researchcontributions have been mainly in the area of nanophotonics.

David Sinton is an Assistant Professor in Mechanical Engineer-ing at the University of Victoria. His research area is in micro- andnanofluidics.

Karen L. Kavanagh is a Physics Professor at Simon Fraser Uni-versity. Her main fields of interest are electronic materials sci-ence and nanofabrication.

Alexandre G. Brolo is an Associate Professor of Chemistry at theUniversity of Victoria. His current interests are on the develop-ment of substrates for enhanced spectroscopy and chemicalsensing.

FOOTNOTES

*To whom correspondence should be addressed. Phone: 1 250 721 7167. Fax: 1 250 7217147. E-mail: [email protected].

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